packet: improve socket create/bind latency in some cases

Most people acquire PF_PACKET sockets with a protocol argument in
the socket call, e.g. libpcap does so with htons(ETH_P_ALL) for
all its sockets. Most likely, at some point in time a subsequent
bind() call will follow, e.g. in libpcap with ...

  memset(&sll, 0, sizeof(sll));
  sll.sll_family          = AF_PACKET;
  sll.sll_ifindex         = ifindex;
  sll.sll_protocol        = htons(ETH_P_ALL);

... as arguments. What happens in the kernel is that already
in socket() syscall, we install a proto hook via register_prot_hook()
if our protocol argument is != 0. Yet, in bind() we're almost
doing the same work by doing a unregister_prot_hook() with an
expensive synchronize_net() call in case during socket() the proto
was != 0, plus follow-up register_prot_hook() with a bound device
to it this time, in order to limit traffic we get.

In the case when the protocol and user supplied device index (== 0)
does not change from socket() to bind(), we can spare us doing
the same work twice. Similarly for re-binding to the same device
and protocol. For these scenarios, we can decrease create/bind
latency from ~7447us (sock-bind-2 case) to ~89us (sock-bind-1 case)
with this patch.

Alternatively, for the first case, if people care, they should
simply create their sockets with proto == 0 argument and define
the protocol during bind() as this saves a call to synchronize_net()
as well (sock-bind-3 case).

In all other cases, we're tied to user space behaviour we must not
change, also since a bind() is not strictly required. Thus, we need
the synchronize_net() to make sure no asynchronous packet processing
paths still refer to the previous elements of po->prot_hook.

In case of mmap()ed sockets, the workflow that includes bind() is
socket() -> setsockopt(<ring>) -> bind(). In that case, a pair of
{__unregister, register}_prot_hook is being called from setsockopt()
in order to install the new protocol receive handler. Thus, when
we call bind and can skip a re-hook, we have already previously
installed the new handler. For fanout, this is handled different
entirely, so we should be good.

Timings on an i7-3520M machine:

  * sock-bind-1:   89 us
  * sock-bind-2: 7447 us
  * sock-bind-3:   75 us

sock-bind-1:
  socket(PF_PACKET, SOCK_RAW, htons(ETH_P_IP)) = 3
  bind(3, {sa_family=AF_PACKET, proto=htons(ETH_P_IP), if=all(0),
           pkttype=PACKET_HOST, addr(0)={0, }, 20) = 0

sock-bind-2:
  socket(PF_PACKET, SOCK_RAW, htons(ETH_P_IP)) = 3
  bind(3, {sa_family=AF_PACKET, proto=htons(ETH_P_IP), if=lo(1),
           pkttype=PACKET_HOST, addr(0)={0, }, 20) = 0

sock-bind-3:
  socket(PF_PACKET, SOCK_RAW, 0) = 3
  bind(3, {sa_family=AF_PACKET, proto=htons(ETH_P_IP), if=lo(1),
           pkttype=PACKET_HOST, addr(0)={0, }, 20) = 0

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 279467b74eb70499586e21528842bc99efa1676e..85bb38cb56fd28acd8b18e95b345da085b2cd7ad 100644 (file)
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -2567,9 +2567,12 @@ static int packet_release(struct socket *sock)
  *     Attach a packet hook.
  */
 
-static int packet_do_bind(struct sock *sk, struct net_device *dev, __be16 protocol)
+static int packet_do_bind(struct sock *sk, struct net_device *dev, __be16 proto)
 {
        struct packet_sock *po = pkt_sk(sk);
+       const struct net_device *dev_curr;
+       __be16 proto_curr;
+       bool need_rehook;
 
        if (po->fanout) {
                if (dev)
@@ -2579,21 +2582,29 @@ static int packet_do_bind(struct sock *sk, struct net_device *dev, __be16 protoc
        }
 
        lock_sock(sk);
-
        spin_lock(&po->bind_lock);
-       unregister_prot_hook(sk, true);
 
-       po->num = protocol;
-       po->prot_hook.type = protocol;
-       if (po->prot_hook.dev)
-               dev_put(po->prot_hook.dev);
+       proto_curr = po->prot_hook.type;
+       dev_curr = po->prot_hook.dev;
+
+       need_rehook = proto_curr != proto || dev_curr != dev;
+
+       if (need_rehook) {
+               unregister_prot_hook(sk, true);
 
-       po->prot_hook.dev = dev;
-       po->ifindex = dev ? dev->ifindex : 0;
+               po->num = proto;
+               po->prot_hook.type = proto;
+
+               if (po->prot_hook.dev)
+                       dev_put(po->prot_hook.dev);
 
-       packet_cached_dev_assign(po, dev);
+               po->prot_hook.dev = dev;
+
+               po->ifindex = dev ? dev->ifindex : 0;
+               packet_cached_dev_assign(po, dev);
+       }
 
-       if (protocol == 0)
+       if (proto == 0 || !need_rehook)
                goto out_unlock;
 
        if (!dev || (dev->flags & IFF_UP)) {